CRYSTAL GROWTH & DESIGN
Forced Migration of Nonsoluble and Soluble Metallic Pollutants ahead of a Liquid-Solid Interface during Unidirectional Freezing of Dilute Clayey Suspensions
2002 VOL. 2, NO. 2 135-140
Guillaume Gay* and M. Aza Azouni Laboratoire des Mate´ riaux et des Structures du Ge´ nie Civil, UMR 113 CNRS/LCPC, Cite´ Descartes, 2 alle´ e Kepler, 77420 Champs-sur-Marne, France Received December 13, 2001
ABSTRACT: To test whether suspended and dissolved forms of metallic pollutants in water can be simultaneously separated by freezing, we subjected water initially at 0 °C and containing (0.69-1.03) × 10-2 mol of dissolved lead nitrate and 0.83-1.68 g of suspended clay (Na-montmorillonite) per kilogram of suspension to unidirectional freezing. Less suspended and dissolved forms of lead remained behind the freezing interface as compared to that in the initial suspension. The greatest reduction of total lead behind the freezing interface was obtained when the pH was between 5 and 9 and when -2 °C cooling was used: i.e., for a mean rate of freezing front advance on the order of 1 µm s-1. Introduction Pollutants in sludges resulting from urban or industrial wastewater remediation can be encountered as nonsoluble (precipitated, adsorbed on colloidal surfaces, ...) and soluble (ionic, complexed, ...) compounds.1,2 The treatment of the first nonsoluble species can certainly be achieved by mechanical separating processes, e.g. sedimentation, flocculation, flotation, or filtration, but the treatment of soluble compounds is more difficult and requires an additional operation. Thus, it is obviously more desirable economically to use only one process to remediate contaminated media where both nonsoluble and soluble pollutants exist. For exploring such a complete process, our idea is to use the principle of freezing separation, which has been extensively used in metallurgy,3-5 chemical analyses,6,7 materials sciences,8 food products, civil engineering,9,10 and, more recently, in cryobiology.11 This method can be favorably applied to nonsoluble and soluble compounds.12 Indeed, during the freezing of a liquid suspension of second-phase particles, interactions between the suspended particles and the liquid-solid interface can be observed.13-15 Particles can be either repelled and swept along by the freezing front or encapsulated in the solid phase. The value of the front velocity vf, at which both the repulsion and the capturing of a foreign particle are possible, is determined as the critical rate vc of the advancing freezing interface.16 The quantity vc depends essentially on the typical dimension R of the particle but is also affected by the conductivity, the geometry, and the surface properties of the particle. In the ideal case of a smooth spherical particle in interaction with a planar freezing front, the critical velocity is expressed as17
vc )
2(Fp - Fl)gRd0 -A 36πηβRd0 9ηβ
(1)
* To whom correspondence should be addressed. Tel: +31 1 40435467. Fax: +31 1 40435485. E-mail:
[email protected].
where A is the Hamaker constant of the ice-waterparticle system (A < 0 in this particular system), η is the dynamic viscosity of the suspension, d0 is the minimal distance between the particle and the freezing front, Fp is the particle density, and g is gravity. The dimensionless parameter β links together the freezing front velocity vf, the relative velocity of the particle vp and the densities of the solid phase Fs and of the liquid phase Fl as follows: β ) vp/vf - (Fl - Fs)/Fl. For nonideal situations, more complicated expressions of vc exist to take into account the effects of the interface shape and the surface properties of the particle.18,19 In all cases, vc decreases with increasing particle size. Thus, one has to impose a very small freezing front rate vf in order to repel a large range of particles. On the other hand, when a liquid solution, initially of uniform composition, is solidified progressively, the composition of the solid is not uniform. The distribution of solute in the solid, when solidification is complete, is different from that in the liquid, although the total amount of solute remains unchanged.20-22 The principle of such segregation is based on the liquid-solid binary diagram phase. It is convenient to consider the idealized case, in which the liquidus and the solidus lines are both straight. This case can be reasonably approached in practice for small values of solute concentration: i.e., on the order of 1%.3 The equilibrium distribution coefficient k0 is the ratio of solute concentrations of the solid and the liquid in equilibrium with it:
k0 ) Cs/Cl
(2)
where Cs and Cl are the solute concentrations at the interface in the solid phase and in the liquid phase, respectively. When the solute solubility in the liquid phase is higher than in the solid phase, k0 < 1 and there is an increase of the solute concentration in the liquid phase. This corresponds to the case of metallic salts. The equilibrium distribution coefficient k0 is rarely observed in a practical freezing process. Far from equilibrium, e.g. in the presence of convection in the liquid phase, it is necessary to consider an effective
10.1021/cg0100383 CCC: $22.00 © 2002 American Chemical Society Published on Web 02/14/2002
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Figure 1. Schematic representation of the experimental cell. Under the thermal gradient imposed by the temperatures at the bottom Tb and at the top Tu of the cell, a solid-liquid interface propagates unidirectionally in the sample leading to the redistribution of the metallic pollutant along the cell.
distribution coefficient, called ke. This effective distribution coefficient is related to the equilibrium distribution coefficient by the equation23
() ke k0
-1
( )
) k0 + (1 - k0) exp -
δDvf D
(3)
where δD is the distance from the interface where convection can be neglected and D is the diffusion coefficient of the solute. According to eq 3, ke depends on vf and gets close to k0 when vf decreases. The dependence of ke on vf can be explained from a microscopic point of view during dendrite formation.5,22 In fact, when fast freezing occurs, dendrites are thin, highly branched, and closely spaced. Consequently, a freeze-concentrated solution, which lies between adjacent dendrites, can hardly escape from this location. Conversely, when slow freezing occurs, dendrites are thick, weakly branched, and largely spaced. A freeze-concentrated solution thus exudes easily from the large channels between adjacent dendrites. Therefore, ke decreases as the freezing rate decreases. One gets the best solute rejection for small values of vf, which fortunately correspond also to the best repulsion rate of nonsoluble particles, as described above. This paper deals with the rejection of metallic pollutants by a freezing front moving unidirectionally in dilute clayey suspensions contaminated by lead. Such media can be considered as models of polluted sludges. This process can be utilized in remediating sludges charged with heavy metals. Apparatus and Experimental Procedure Experimental Cell. Unidirectional freezing experiments were carried out in a cell constructed from Plexiglas (good heat insulator). The test section was rectangular, with a cross section of 80 mm × 60 mm and a depth of 20 mm. The Plexiglas walls were 10 mm thick. The top and the bottom of the cell were made of smooth brass plates, which could be cooled to preselected temperatures. Thermocouples T (copper-nickel-copper) were inserted into holes in the brass plates. These thermocouples were connected to a recorder, and their accuracy was not better than 0.05 °C. The beginning of freezing could be exactly determined by detecting on the temperature recording the moment at which
undercooling stops. This recording also allowed us to control the stability of the temperatures during runs and to ensure a uniform cooling. Figure 1 represents the experimental cell and the presumed variations of temperatures and metallic concentrations in the solid and liquid phases. Optical Device. The employed freezing process yielded an ice-water front, which propagated with decreasing velocity. An optical device was used in order to determine at all times the position of the freezing front. The various phases of the freezing process were observed using a shadowgraph. In our experimental setup, the cell was oriented so that the short side was along the optical axis. The shadowgraph technique relies upon the variation of the refractive index of a fluid with its density and, thence, its temperature. The obtained images were video recorded and processed by a computing system with a high time resolution. Figure 2 represents such an arrangement. Samples. Experiments were performed with either aqueous solutions of lead or dilute clayey suspensions charged with lead. Since nitrate salts dissolve completely and since nitrate ions react very little, lead was introduced as nitrate. Lead concentrations varied from 0.69 × 10-2 mol to 1.03 × 10-2 mol per kilogram of the suspension. The clay used was a Na-montmorillonite containing few traces of silica and carbonates and very few traces of heavy metals. Table 1 sums up the main characteristics of this clay mineral.24 Clay concentrations varied from 0.83 to 1.68 g per kilogram of the suspension. When reported to the kilogram of dry matter, lead concentrations varied from 94 to 720 g kg-1, which corresponded to high pollution levels in comparison with typical sludges issued from wastewater treatment.25,26 Accounting for the cationic exchange capacity of the clayey mineral, there were between 6.3 and 21.6 times more metallic cations than strictly needed to occupy all the clayey sites, when the physicochemical conditions allowed such an adsorption. Experimental Procedure. Before the beginning of each run, the cell was carefully filled with the studied medium (aqueous solution of lead, clayey suspension charged with lead) by the means of an ad hoc device,24 which avoided the formation of macroscopic bubbles. Then, the upper and bottom plates were maintained over 12 h at the same positive temperature around 0 °C, so that the medium was at a uniform temperature, in a state of equilibrium, at the beginning of each run. In the meantime, with the optics in close alignment and the cell in place, the final adjustments were made. Then, a constant negative temperature Tb was applied to the lower part, while the top was maintained at the initial
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Crystal Growth & Design, Vol. 2, No. 2, 2002 137
Figure 2. Complete optical device: shadowgraph arrangement, video recording, and computer processing. The dotted lines represent the routes of light beams. The components are not to scale. Table 1. Main Characteristics of the Na-Montmorillonite dry density equilibrium pH cationic exchange capacity isoelectric point carbonate content total traces of heavy metals (lead, zinc, copper, ...)
2144 kg m-3 10.3 116 cmol kg-1 4.3 ∼1% ∼325 mg kg-1
temperature Tu ≈ 0 °C and the timer of the video system was triggered off. For the set of experiments described here, Tb ranged from around -15 to -2 °C. Freezing was stopped when around a third of the sample was solidified. To determine the concentrations of lead and of clay in the solidified and the nonsolidified phases, the two phases were carefully separated at the end of each run: the liquid phase was aspired, and then the upper surface of the solid slab was washed with a known volume of distilled water to be sure to get all the solute that was rejected by the freezing front, including solute accumulated at the interface. Then, video recording was analyzed to provide the variations of the freezing front position versus time. Determination of Concentrations. Total lead concentrations, i.e., without distinguishing the chemical forms, were measured by an atomic absorption spectrophotometer after extracting lead from clay particles by an acid attack. By this method, lead concentrations were given with an uncertainty of around 1%. Clay concentrations were evaluated by titration with methylene blue, which is a precise method when there is only a small quantity of clay to measure.27,28 Clay concentrations were determined with an uncertainty between 1 and 2%. For Cinitial being the initial concentration of one component, lead or clay, and SCfinal being the final concentration of the same component in the solid phase, we define the rate of purification by the ratio
φ)
Cinitial - SCfinal Cinitial
(4)
This definition facilitated comparison from one run to another. When φ ) 0, the measured component was completely entrapped in the solid phase. For φ > 0, at least a part of the component was rejected. Total purification occurred for φ ) 1. Analysis of Video Recording. On the obtained images, the front surface appeared flat, as is depicted in Figure 3, so that the position zf of the freezing front could be easily determined at each instant t. After establishing the fitting
Figure 3. The solid phase (in gray in the lower part) and the liquid phase (in white in the upper part) as visualized on the frosted screen by the CCD camera. The solid line is drawn to show the interface. curve zf(t) (least-squares fit), one could deduce the evolution of the velocity vf(t) by deriving zf(t).
Results and Discussion Validation of the Unidirectional Freezing. At first, we need to check that the propagation of the freezing front is really one-dimensional in our experimental cell. Let us apply the temperature Tb ) - 2.0 °C to the bottom of the cell and follow the propagation of the freezing front zf(t) in the cases of pure distilled water, an aqueous solution of lead, and a dilute clayey suspension charged with lead. As illustrated in Figure 4, the position and the velocity of the freezing front evolve similarly for the three cases. This is not surprising, as we have used small concentrations for lead and for clay. On the other hand, the fitted curve of zf(t) agrees with Neumann’s solution for the unidirectional phase change problem, which stipulates that the position of the freezing front varies as29
zf ) 2λ(κst)0.5
(5)
where κs is the thermal diffusivity of the solid phase and λ is a numerical constant. In the present freezing situation, the liquid phase was initially at almost its
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Figure 4. Fitted curves of the position (a) and the velocity (b) of the freezing front versus time for pure distilled water (+), aqueous solution of lead (O), and clayey suspension charged with lead (*).
melting point Tf ) 0 °C. In such a case, λ can be approximated by29
csp(Tf - Tb) λ ≈ 2Lf 2
(6)
where csp is the specific heat of the solid phase at constant pressure and Lf is the latent heat of the liquidsolid phase change. With the assumption that the melting point is not significantly affected by the small concentrated impurities, if we introduce eq 6 in eq 5, we get
zf )
(
2Ks|Tb| t FsLf
)
0.5
(7)
where Ks is the thermal conductivity of the solid phase. For pure distilled water, one has Fs ) 919.49 kg m-3, Ks ) 2.218 J m-1 s-1 K-1, and Lf ) 333 188 J kg-1. Thus, for Tb ) - 2.0 °C, the theoretical evolution of the freezing front position is theoretical
zf
) (1.70 × 10-4)t0.5
(8)
The comparison of the plot of eq 8 with the evolution resulting from the fit (with a correlation coefficient verifying r2 ) 0.9997) of the experimental results concerning pure distilled water (eq 9) shows that the two curves are very close, as is shown in Figure 5.
) (1.85 × 10-4)t0.48 zfitted f
(9)
This confirms that the heat flow was really onedimensional in the experimental cell. Influence of the Cooling Rate on the Purification Rate φ. Experiments were carried out on aqueous solutions of lead for a given concentration with different values of Tb: -15.1, -9.6, -5.2, and -2.0 °C. Table 2 gives the initial and the final characteristics of these solutions. For the four cases, one can demonstrate that lead occurs totally in the chemical form of dissolved salts.
Figure 5. Measured positions of the freezing front versus time for pure distilled water (( standard deviation). The dotted line shows the fitted curve given by eq 9. The solid line shows the analytical solution given by eq 8. Table 2. Influence of the Temperature Tb at the Bottom of the Cell on the Purification Rate φ of Lead lead concn (10-2 mol kg-1) Tb (°C)
init pH
init
final in the solid phase
φ (%)
-15.1 -9.6 -5.2 -2.0
3.9 4.1 4.3 3.9
1.03 1.02 1.02 0.93
1.00 0.98 0.91 0.62
3.2 4.2 11.0 33.8
When Tb varies from -15.1 to -2.0 °C, we notice that φ increases from 3.2 to 33.8%. It is obvious that the change of Tb affects the evolution of the freezing front velocity, as is shown when eq 7 is derived:
vf )
(
)
Ks|Tb|1 2FsLf t
0.5
(10)
With decreasing |Tb|, the freezing front propagates more slowly. According to eq 3, this leads to a more
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Crystal Growth & Design, Vol. 2, No. 2, 2002 139
Table 3. Initial Characteristics of the Dilute Clayey Suspensions Charged with Lead and Final Results after Freezing One-Third of the Samples init concn
final concn in the solid phase
purification rate (%)
init pH
lead (10-2 mol kg-1)
clay (g kg-1)
lead (10-2 mol kg-1)
clay (g kg-1)
lead
clay
3.0 4.0 4.8 5.4 6.0 7.5 9.0 10.0
0.86 0.92 0.92 0.69 0.97 0.93 0.97 0.96
1.68 1.22 1.51 1.39 0.94 0.83 1.08 1.52
0.82 0.74 0.64 0.54 0.66 0.63 0.58 1.04
1.96 0.79 1.20 1.10 0.40 0.61 0.70 0.89
4.6 19.7 29.8 22.2 31.5 32.4 40.4 -7.5
-16.1 35.1 20.6 21.1 57.0 26.6 35.1 41.6
Therefore, to obtain the best purification rate of the metallic pollutant, it is necessary to work between pH 5.0 and 9.0, where both metallic pollutant and clay are effectively rejected. Let us recall that metallic pollutants (lead in the present case) may exist in several chemical forms,1,2 particularly dissolved and/or adsorbed on clay surfaces. For the extreme values of pH (pH 3.0 and 10.0), lead or clay occurs in chemical speciations, which deposit in the bottom of the cell over the period of 12 h needed to attain the state of equilibrium. Indeed, it can be proved that lead forms numerous precipitates with hydroxides at pH 10.0 and that clay particles form some aggregates at pH 3.0, which is less than the isoelectric point pHIEP 4.3.31,32 Conclusion
Figure 6. Purification rates for lead (9) and clay (b) versus pH. The lines are only used to guide the eye.
effective segregation and then to a more important purification rate, as was just observed above. Influence of the pH on the Purification Rates. Dilute clayey suspensions charged with lead were submitted to unidirectional freezing for various values of pH between 3.0 and 10.0. To reject the maximum of the metallic pollutant, all the following experiments were carried out at Tb ) -2 °C, which corresponded to a freezing front velocity varying from 10 µm s-1 at the beginning of the freezing to 0.5 µm s-1 at the end of the run, with an average value around 1 µm s-1. All the initial and the final characteristics of these runs are summarized in Table 3. We note that purification rates are influenced by the pH and the concentrations of lead and clay. Nevertheless, when the initial concentrations differ only from small values, as is the case in Table 3, we made it evident that initial concentrations had no influence on the purification rates in comparison with the pH effect.30 The predominant influence of pH is pointed out in Figure 6, where purification rates of lead and clay versus pH are displayed. The purification rate of lead, φlead, is almost zero at very acid pH. With increasing pH, φlead increases slowly until it reaches a stable level at 20% e φlead e 40% for 5.0 e pH e 9.0. Beyond pH 9.0, φlead decreases and becomes negative at pH 10.0. Regarding clay, its purification rate φclay is negative at pH 3.0 and then increases quickly to 35% in only one unit of pH. With pH 4.0 as a starting point, φclay oscillates around a mean value of 34%.
Freezing separation seems to be a promising method to remediate wastewater contaminated by heavy metals. By using a new device at a small scale, we showed that a freezing front moving slowly, with an average velocity around 1 µm s-1, was practically able to reject dissolved metallic salts and to repulse heavy metals bound to nonmiscible particles such as clay colloids or undissolved salts. The main advantage of this method is its great independence from the chemical forms of heavy metals. Moreover, we pointed out the major role played by the pH. A way to improve the obtained purification rates is to have recourse to several freeze-thaw cycles. As illustrated by Pfann4 in the case of the zone melting, successive freeze-thaw cycles lead to the best segregation of the soluble compounds. Further experiments will be carried out to confirm the usefulness of multiple freeze-thaw cycles in rejecting both nonsoluble and soluble compounds. Relying on the recent studies on the redistribution of solutes in a porous medium submitted to unidirectional freezing,33 we also plan to extend our experimental work to a model of soils polluted by heavy metals, consisting of porous packs saturated by contaminated clayey suspensions. Acknowledgment. We gratefully acknowledge valuable discussions with Professor Bernard Billia. The measurements of lead concentrations were performed in the Centre de Ge´ologie de l’Inge´nieur (Marne-laValle´e), thanks to the help of Caroline Lapeyre. References (1) Stumm, W.; Morgan, J. J. Aquatic Chemistry: An Introduction Emphasizing Chemical Equilibria in Natural Water, 2nd ed.; Wiley: New York, 1981. (2) Evans, L. J. Environ. Sci. Technol. 1989, 23, 1046-1056. (3) Tiller, W. A.; Jackson, K. A.; Rutter, J. W.; Chalmers, B. Acta Metall. 1953, 1, 428-437. (4) Pfann, W. G. Zone Melting; Wiley: New York, 1958. (5) Tiller, W. A. In The Art and Science of Growing Crystals; Gilman, J. J., Ed.; Wiley: New York, 1963; Chapter 15, pp 276-312. (6) Shapiro, J. Science 1961, 133, 2063-2064. (7) Baker, R. A. Water Res. 1967, 1, 61-77. (8) Chuvilin, E. M.; Erchov, E. D.; Murashko, A. A. In Proceedings of the International Conference on Permafrost and Actions of Natural or Artificial Cooling; Orsay, France; International Institute of Refrigeration: Paris, 1998; pp 3440.
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(9) Shiina, M.; Kashiwagi, T.; Kitamura, Y. In Proceedings of the International Symposium on Ground Freezing and Frost Action in Soils; Luleå, Sweden; Balkema: Rotterdam, 1997; pp 401-405. (10) Berggren, A.-L. In Proceedings of the International Symposium on Ground Freezing and Frost Action in Soils; Louvainla-Neuve, Belgium; Balkema: Rotterdam, 2000; pp 267272. (11) Otero, L.; Martino, M.; Zaritzky, N.; Carrasco, J. A.; De Elvirac, C.; Sanz, P. D. In Proceedings of the International Conference on Permafrost and Actions of Natural or Artificial Cooling; Orsay, France; International Institute of Refrigeration: Paris, 1998; pp 249-252. (12) Halde, R. Water Res. 1980, 14, 575-580. (13) Corte, A. E. J. Geophys. Res. 1962, 67, 1085-1090. (14) Chernov, A. A.; Temkin, D. E. In First European Conference on Crystal Growth and Materials Symposium; Zurich, Switzerland; North-Holland: Amsterdam, 1977; pp 4-77. (15) Ko¨rber, C.; Rau, G. J. Cryst. Growth 1985, 72, 649-662. (16) Azouni, M. A.; Kalita, W.; Yemmou, M. J. Cryst. Growth 1990, 99, 201-205. (17) Casses, P.; Azouni, M. A. Adv. Colloid Interface Sci. 1994, 50, 103-120. (18) Azouni, M. A.; Casses, P. Adv. Colloid Interface Sci. 1998, 75, 83-106. (19) Rempel, A. W.; Worster, M. G. J. Cryst. Growth 2001, 223, 420-432. (20) Rutter, J. W.; Chalmers, B. Can. J. Phys. 1953, 31, 15-39. (21) Smith, V. G.; Tiller, W. A.; Rutter, J. W. Can. J. Phys. 1955, 33, 723.
Gay and Azouni (22) Chalmers, B. Principles of Solidification; Wiley: New York, 1964. (23) Burton, J. A.; Prim, R. C.; Slichter, W. P. J. Chem. Phys. 1953, 21, 1987-1996. (24) Gay, G. Application du froid artificiel au traitement des boues et des sols pollue´s par des me´taux lourds, the´orie et expe´riences a` petite e´chelle sur des milieux mode`les. Ph.D. Thesis, Ecole Nationale des Ponts et Chausse´es, Marne-laValle´e, France, 2001. (25) Baron, J. Technical Report EG7; Laboratoire Central des Ponts et Chausse´es, Paris, 1991. (26) Alloway, B. J. Heavy Metals in Soils, 2nd ed.; Blackie: Glasgow, Scotland, 1995. (27) Hang, P. T.; Brindley, G. W. Clays Clay Miner. 1970, 18, 203-212. (28) Kahr, G.; Madsen, F. T. Appl. Clay Sci. 1995, 9, 327-336. (29) Carslaw, H. C.; Jaeger, J. C. Conduction of Heat in Solids, 2nd ed.; Oxford Science: New York, 1959. (30) Gay, G.; Azouni, M. A. Polar Rec. 2001, 37, 257-263. (31) Overbeek, J. T. G. In Colloid Science; Kruyt, H. R., Ed.; Elsevier: Amsterdam, 1952; Vol. 1, Chapter IV, pp 115193. (32) Hunter, R. J. Zeta Potential in Colloid Science: Principles and Applications; Academic Press: London, 1981. (33) Watanabe, K.; Muto, Y.; Mizoguchi, M. Cryst. Growth Des. 2001, 1, 207-211.
CG0100383
